1. Field of the Invention
The present invention relates generally to medical X-ray imaging and in particular to a method for determining spatial resolution and noise level in medical projection X-ray imaging.
2. Description of the Related Art
Projection X-ray imaging is used in a medical setting to image structures within the body of a patient. The medical projection X-ray imaging device must be set up to provide a compromise between spatial resolution and noise level. For example, a large focal spot size guarantees a high quantum flux and thus a reduced noise level. On the other hand, image resolutions suffers from a poorly conditioned source. Duration of the exposure also determines image quality since motion of an object during the exposure results in blurring of the object image if the exposure time is too long. On the other hand, short pulses or exposure times reduce the motion blurring effect but lead to high noise levels.
Issues of spatial resolution, contrast, and noise are addressed by the following methods.
In CNR (contrast to noise ratio) or SDNR (signal difference to noise ratio) techniques, the two noted parameters combine contrast and noise.
In MTF (modulation transfer function) techniques, the function describes the loss of modulation amplitude for every spatial frequency, which is caused by the imaging system.
In DQE (detective quantum efficiency) techniques, the function describes the detected performance of a detector compared to an ideal detector, which is in reference to spatial resolution, contrast and noise.
In NPS (noise power spectrum) techniques, the frequency-dependent noise level is considered.
The exposure parameters on an X-ray system are usually chosen in such a way as to ensure a constant detector dose by an automatic exposure control system (AEC). The properties of the object being imaged, such as the speed of an object in motion, are not taken into account.